The present invention relates to inspection mirrors. Such mirrors are useful in inspecting printed circuit boards and like electronic components where visual access to a part or parts is restricted. Such mirrors also find application in medical settings. Dentists, in particular, use inspection mirrors to examine the teeth and to guide instruments and drill bits during procedures. A primary consideration for all inspection mirrors is the quality of the image reflected from the mirror surface. Image quality is important in direct viewing and in photography or other imaging of the surface being inspected.
The use of inspection mirrors in the inspection of printed circuit boards is described in the inventor's prior U.S. Pat. No. 4,379,647, entitled Optical Comparator and Inspection Apparatus; U.S. Pat. No. 4,938,579, Side-Viewing Mirror Device and U.S. Pat. No. 4,795,237 entitled Optical Inspection Method with Side-Viewing Mirror.
Prior art inspection mirrors have been extremely delicate and susceptible to damage. Frameless mirrors are particularly vulnerable when the substrate contacts the object under inspection. The susceptibility of prior art mirrors to damage is, in part, a result of the use of front surface metalization. Rear surface mirrors have been employed in inspection settings, but they have not proved durable either because the unprotected substrate is susceptible to damage by scratching, cracking and scoring as the mirror comes in contact with the parts under inspection. When thick substrates are employed, a degradation of the reflected image occurs.
It is therefore desirable to have an inspection mirror which provides a high-quality image while at the same time protecting the mirror from damage. Such a mirror is particularly desirable where the total thickness of the mirror and the necessary supporting structure is maintained sufficiently thin that the mirror can provide visual access to even the most restricted spaces under inspection.